Thinly slice the onion. Place a frying pan over a medium-low heat and cook the onion until soft, golden, and caramelised, stirring regularly.
While the onion cooks, mix the Dijon mustard, fat free yoghurt, salt, and black pepper in a small bowl to make the sauce. Set aside.
Season the sirloin steak on both sides with the salt, black pepper, and garlic granules.
Once the onion is cooked, remove it from the pan and set aside.
Increase the heat to medium-high and cook the steak in the same pan until done to your liking.
Add the slice of reduced fat cheese on top of the steak for the final minute so it melts.
Slice the ciabatta roll open. Spread the lighter than light mayonnaise on one half and the Dijon yoghurt sauce on the other.
Fill the ciabatta with rocket, the caramelised onion, and the cheesy steak.
Serve and enjoy!